Absolutely not cumulative.

9 means at least 9 seats available in the fare bucket (so could be 34) but equally could be 9. Also, with only 12 economy fare buckets × 9 you can only get to 108...

In the case of the fare bucket, you're right that if they sold four of the J8, it'd reduce to J4 but they could immediately decide to open up more. Similarly, someone could buy 4 of the W seats as well, sell out W, reducing the number of J seats by 4 as well (because BA expects all of those W passengers either to be upgraded or to cause someone else to be). The same goes for any of the cheaper business fares and in each case, BA could decide to open up more seats if they now expect more cancellations (selling fully flex), can afford to downgrade, or for any other reason in response to changing conditions.

Willingness to sell doesn't necessarily mean a seat on board, either - it means willingness to take (a lot of) someone's money and offload and compensate someone else if needed (for much less)!